The present invention relates to the art of digital loop detectors and more particularly to a digital loop detector with an automatic tuning circuit.
The invention is particularly applicable for a digital loop detector to be used for detecting vehicles traveling along a roadway and it will be described with particular reference thereto; however, it is appreciated that the invention has much broader applications and may be used in various digital loop detectors for a variety of uses.
The invention is particularly applicable for a digital loop detector used in detecting the presence of transitory vehicles entering and/or leaving a selected area or zone in, or associated with, a traffic intersection or selected area in a roadway. For a number of years, traffic control systems for controlling vehicle movement through intersections have been controlled, to various extends, by detectors which detect the number of vehicles entering the vicinity of the intersection. At first, these detectors were pressure treadles and other devices mechanically actuated by a passing vehicle. Such detectors were susceptible to deterioration and climate variations; therefore, other detectors were developed which did not function on mechanical principles. These involved radar sensors, magnetic pick-ups, solar units and induction loop pick-ups. Of these, one of the most predominately used mechanisms was the induction loop controlled detector. In this device, a loop is embedded in a roadway adjacent the intersection and the inductance of the loop is affected by vehicles entering and leaving the field of influence of the loop. In initial systems, the loop was connected to an oscillator. Changes in the frequency, and/or amplitude, of the oscillator output were analyzed by analog circuits for indicating a vehicle detection. Although extensively used, this type of analog system was extremely sensitive and varied substantially with the ambient conditions. In an attempt to diminish the effect of ambient conditions, some systems were provided with a fixed frequency oscillator which was beat with the loop oscillator to produce an analog signal representative of the presence of metal within the field of influence of the detecting loop. These systems did not prove extremely satisfactory because the ambient conditions still affected the operation of the loop, irrespective of the presence of metal in the field of influence. Thus, these systems often involved a feedback tuning arrangement for the fixed frequency oscillator which attempted to bring the loop oscillator and reference oscillator into a controlled relationship preparatory to detection. The feedback tuning arrangements for the fixed frequency oscillator presented difficulties which were compounded by the analog nature of the total detector system.
These disadvantages of prior attempts to use a loop detector were overcome by a digital detecting arrangement disclosed in U.S. Pat. No. 3,868,626. In this prior patent, two oscillators are employed. One oscillator has a fixed frequency and the other oscillator has a frequency varied according to the condition of the detector loop. This type of system creates a pulse count during a counting interval. Detection can be recorded either by varying the counting interval by the loop controlled oscillator or by varying the rate or frequency of the count pulse created by the loop controlled oscillator. In either arrangement, when the pulse count of a given counting interval differs from the reference count, by a threshold number or magnitude, detection is indicated. These systems have been quite successful in overcoming the disadvantages of prior systems for using a loop to detect the presence of a vehicle adjacent an intersection. The oscillators do not require periodic tuning with respect to each other during the continued operation of the detector. The detection is positive and the sensitivity of the detector system can be controlled by changing the threshold number or magnitude. The present invention relates to an improvement in a digital loop detector, as shown in U.S. Pat. No. 3,868,626, which improvements involve an automatic tuning network wherein the magnitude of the count pulse during a counting interval can be varied to maintain a relatively high number of counts. This increases the sensitivity of the digital detector. In accordance with the present invention, the general magnitude of the pulse count during a counting interval can be retained in a general range which range is sufficiently high to maintain a high sensitivity for the operation of the digital loop detector. Prior U.S. Pat. No. 3,868,626 is incorporated by reference herein as background information.